Variable annular illuminator for photolithographic projection imager

ABSTRACT

The illumination of a photolithographic projection imager is given a variable annular intensity profile by using diverging and counter diverging elements that are movable relative to each other in the illumination path. An upstream element diverges the illumination into an annular configuration, the radius of which is set by the distance downstream to the counter diverging element. Convex and concave conical surfaces on the movable elements can accomplish this.

RELATED APPLICATIONS

This application is a Continuation of parent application Ser. No.08/342,787, filed 21 Nov. 1994, by Paul G. Dewa, Paul F. Michaloski,Paul J. Tompkins, and William N. Partlo, and entitled VARIABLE ANNULARILLUMINATOR FOR PHOTOLITHOGRAPHIC PROJECTION IMAGER, now U.S. Pat. No.5,452,054, which is continuation of application Ser. No. 08/024,651,filed 1 Mar. 1993, now abandoned.

FIELD OF THE INVENTION

This invention involves illumination systems for microlithographicprojection imagers.

BACKGROUND

Off-axis illumination in an annular configuration has increasedresolution and depth of focus for photolithographic projection imagersimaging a mask or reticle having dense lines and spaces and densecontacts. This is explained in a paper entitled "0.25 μm LithographyUsing a Deep-UV Stepper with Annular Illumination", authored by A. Yen,W. N. Partlo, R. W. McCleary, and M. C. Tipton, and presented atMicrocircuit Engineering 92, Erlangen, Germany, in June of 1992, andwill be published in "Proceedings of Microcircuit Engineering 92",Erlangen, Germany. The advantage of annular illumination involves thediffractive effects of a reticle having dense lines and spaces and densecontacts, and an annular profile of illumination intensity is especiallyadvantageous when imaging a reticle that has a region of densely packedfeatures.

The simplest way to provide annular illumination of a mask or reticle ina photolithographic projection imager has been with a mask arranged atthe pupil of the illuminator to block out the central region of theillumination and leave an annular configuration of illuminationintensity extending around the masked out central portion. This has theserious disadvantage of reducing the total illumination energy availableand thereby slowing down the imaging process.

SUMMARY OF THE INVENTION

We have discovered a way of profiling the illumination for aphotolithographic projection imager to produce an annular configurationof illumination intensity without wasting illumination energy. Ourpreferred way of doing this is also readily variable so that the radiusof the annular illumination profile can be varied quickly andconveniently. Besides having this capacity, our illuminator is alsopreferably able to produce illumination having a standard, non-annularconfiguration. Moreover, our invention implements these capabilities ina reliable and effective way that an operator can control.

The way our invention accomplishes this is by a diverging element in theillumination path of the illuminator followed by a counter divergingelement arranged at a preferably variable distance. The divergingelement is preferably refractive and preferably has a conical surfacearranged for diverging the illumination into an annular configuration ofintensity. The counter diverging element is also preferably refractive,with a conical surface; and it counters the divergence of theillumination and fixes the radius of the annular configuration ofillumination intensity as a function of the distance between the twoelements. Moving the elements apart enlarges the radius of the annularprofile of the illumination, and moving the elements into proximityrestores the illumination to an undiverged condition so that theillumination intensity profile does not become annular.

DRAWINGS

FIG. 1 is a schematic diagram of a microlithographic projection imagerincluding an objective imaging system and an illuminator with an opticalsystem capable of varying an annular intensity profile of theillumination according to the invention.

FIGS. 2 and 3 are schematic diagrams of preferred elements of anilluminator optical system that can vary the illumination intensityprofile between an annular configuration as shown in FIG. 2 and anon-annular configuration as shown in FIG. 3.

FIGS. 4 and 5 are schematic computer models of pupil illuminationintensity profiles corresponding respectively to FIGS. 2 and 3.

FIGS. 6 and 7 are images of pupil illumination intensity profiles at thepupil of the illuminator corresponding respectively to the illuminationintensity profile variations of FIGS. 2 and 3 and to FIGS. 4 and 5.

FIG. 8 is an image of a pupil illumination intensity profile that isintermediate to ones shown in FIGS. 6 and 7.

FIGS. 9 and 10 are schematic views of concentric diffractive elementsproducing divergence and counter divergence and arranged for forming avariable annular illumination intensity profile according to theinvention.

FIG. 11 schematically shows divergent and counter divergent elementsformed of faceted refractive elements for practicing the invention.

FIG. 12 schematically shows a rotatable element carrying different sizemasks movable into the pupil region of the inventive illuminator.

FIG. 13 schematically shows a pair of conical reflective elementsarranged for producing variable illumination divergence according to theinvention.

FIG. 14 shows the conical reflective elements of FIG. 13 adjusted to aposition reducing the radius of the annular illumination intensityprofile.

DETAILED DESCRIPTION

A microlithographic projection imager 10 suitable for variable annularillumination according to the invention is schematically illustrated inFIG. 1. It includes an illuminator 20 and an objective imaging system50, both preferably controlled by a computer 60. Light or radiation froma source not indicated is directed through an optical system ofilluminator 20 to a reticle plane R where a mask or reticle to be imagedis positioned. Illumination passing through a mask at reticle plane Rproceeds through objective imaging system 50 to wafer plane W where themask is imaged by the transmitted illumination.

Projection imaging can vary with the masks or reticles involved, andreticles that have dense lines and spaces or dense contacts canespecially benefit from illumination having an annular intensityprofile. Since all reticles do not have dense features, we prefer thatilluminator 20 be able to deliver illumination having a standardcircular intensity profile that can be used when an annular intensityprofile is not desired. We also prefer that the radius of an annularintensity profile be variable for best accommodating the illumination todifferent configurations of reticles. Finally, we prefer thatilluminator 20 satisfy all these conditions without wasting illuminationenergy, which would slow down the imaging process.

A portion of the optical system of illuminator 20 is schematicallyillustrated in FIG. 2 to show a preferred way of giving the illuminationan intensity profile in an annular configuration. An illumination sourceschematically represented as point 21 provides the illumination to beprofiled and consists of several components that are not illustrated andare not important to the way the illumination is given an annularintensity profile. Suitable source components are also generally known.

Downstream of source 21 is preferably a lens or lens group 22 followedby variable annular illumination intensity profiler 25, whose operationwill be explained. Downstream of annular profiler 25 is another lens orlens group 23, a uniformizer 24, and a lens or lens group 26. The pupilregion 30 of illuminator 20 is downstream of lens group 26, and a mask31 can be positioned in pupil region 30 as explained below. Downstreamof pupil 30 is a lens or lens group 27, a folding mirror 28, and a finallens or lens group 29. Members 22, 23, 26, 27, and 29 can be variedconsiderably according to generally known optical design. Uniformizer 24is preferably a kaleidoscopic rod, but can also be a fly's eye lensformed of an array of a multitude of tiny lenses. It segments theillumination and rearranges the segments to ensure uniformity of theillumination throughout the illumination field, which can be varied fromapproximately circular to different sizes of annuli by means of profiler25. The illumination provided with a variable intensity profile isdelivered from member 29 to a mask or reticle at reticle plane R andthen through objective imaging system 50 to image the reticle--often inreduced size--on a wafer at wafer plane W.

One preferred embodiment of profile varier 25, as shown in FIG. 2, hasthe form of a pair of refractive elements 35 and 36 with confrontingconical surfaces. A concave conical surface of element 35 diverges theillumination into an annular configuration of intensity having a radiusthat increases with distance downstream from element 35. A convexconical surface of element 36 counters the divergence caused by element35 to fix the radius of the annular intensity profile. Elements 35 and36 are preferably movable relative to each other, preferably by movingone or the other, so that the distance between them is variable. Theradius of the annular divergence of the illumination is then a functionof the distance between elements 35 and 36. As shown in FIG. 3, whenelements 35 and 36 are moved into proximity, the illumination divergencecaused by element 35 is countered by element 36 before any substantialdivergence occurs so that the illumination retains its standard,approximately circular profile. With elements 35 and 36 proximate toeach other, profiler 25 is effectively eliminated from the opticalsystem of illuminator 20, which then can be used for imaging reticleswith illumination having a standard circular profile.

Several different types of elements can be arranged to serve as profiler25. For elements 35 and 36 that are illustrated in FIG. 2, the concaveand convex conical surfaces preferably have the same conic angle. Thesesurfaces are separated by an air gap, and preferably no optical elementis positioned between elements 35 and 36. When moved into the proximateposition shown in FIG. 3, the conic surfaces of elements 35 and 36 canbe separated by a millimeter or so.

Diffractive elements 33 and 34 can also serve in profiler 25, as shownin FIGS. 9 and 10. The diffractive features of elements 33 and 34 areformed as concentric circles arranged so that element 33 divergesillumination annularly, and element 34 counters the divergence ofelement 33. This forms an annular illumination intensity profile whenelements 33 and 34 are spaced apart, as shown in FIG. 9, andsubstantially no annular divergence when elements 33 and 34 are movedinto proximity, as shown in FIG. 10.

Another form of varier 25 is refractive elements 37 and 38 that areformed with faceted surfaces, rather than single conical surfaces. In away that is similar to the function of conic surfaced elements 35 and36, elements 37 and 38 respectively diverge and counter diverge theillumination passing through so that varying the distance betweenelements 37 and 38 varies the radius of an annular profile ofillumination intensity, as shown in FIG. 11.

A pair of conic surfaced reflective elements 41 and 42 can also serve asprofiler 25, as shown in FIGS. 13 and 14. Convex conical mirror 41diverges the illumination into an annular configuration that iscountered by conical mirror 42. Again, the radius of the annular profilevaries with the distance between movable elements 41 and 42. This radiuscannot be reduced to near zero, as it can by proximity positioning theelements 33, 34 or 35, 36 or 37, 38; but under some circumstances thismay not be a disadvantage.

The illumination is preferably collimated or approximately collimated inthe region of profiler 25 so that illumination that is approximatelyparallel with the optical axis is diverged by the diverging element andis restored to parallelism with the optical axis by the counterdiverging element. Other angular variations can be introduced into theray paths and the divergence and counter divergence paths, but thismakes the design of profiler 25 unnecessarily complex.

Movement of one of the elements of profiler 25 is preferably undercontrol of computer 60 for operation from a keyboard. Then adjustmentsin the radius of an annular intensity profile or the absence of anannular profile can be made quickly and conveniently.

The annular intensity profile imposed on the illumination by profiler 25appears at the pupil region 30 of illuminator 20 where a generallyunilluminated gap occurs in the center of an illuminated annulus. Toensure that the unilluminated center of the annular profile remainsdark, mask 31 is preferably positioned on the optical axis in pupilregion 30. To accommodate annular intensity profiles that vary inradius, the size of mask 31 also preferably varies in radius. This canbe accomplished by mounting a number of masks 31a-d on a rotary element32, as shown in FIG. 12. Turning element 32, preferably under control ofcomputer 60, then rotates any desired one of the different size masks31a-d into the center of pupil region 30. A variable iris can also bepositioned in pupil region 30 to mask the outer periphery of theillumination passing through. Since most of the illumination is given anannular profile by varier 25, very little illumination is lost by theuse of central and peripheral masks around the inside and outside of theannular intensity profile in pupil region 30.

The appearance of the annular illumination profile in pupil region 30 isshown in the schematic computer modelings of FIGS. 4 and 5. An annularintensity profile is shown in FIG. 4, corresponding with the profilerposition of FIG. 2. The circular standard illumination profile shown inFIG. 5 corresponds to the position of profiler 25 shown in FIG. 3. Thesegmented appearance of the illumination is caused by uniformizer 24.Introduction of mask 31 can eliminate the small amount of strayillumination appearing within the annular profile in FIG. 4.

An image of the annular illumination profile produced by the FIG. 2position of profiler 25 is shown in FIG. 6. This is produced by pinholeillumination of a video camera arranged downstream of reticle plane R, Anon-annular illumination intensity profile is shown in FIG. 7,corresponding to the position of profiler 25 shown in FIG. 3. The FIG. 8image, made in the same way as the images of FIGS. 6 and 7, shows asmaller radius of annular illumination intensity caused by a profilerposition between the ones shown in FIGS. 2 and 3.

We claim:
 1. A combination of an illuminator and a photolithographicprojection imager, the combination comprising:a. an illuminator opticalsystem for directing illumination from a source to a pupil of theilluminator from which a reticle is illuminated to be imaged on a waferby an objective imaging system; b. the illuminator in a collimatedregion of illumination upstream of the illuminator pupil having a pairof refractive elements having conical surfaces that are respectivelyconcave and convex; c. said elements being arranged so that an upstreamone of said elements diverges the illumination into a single beam havingan annular configuration of intensity and a downstream one of saidelements counters the divergence caused by the upstream element, to givethe illumination an annular intensity profile of the single beam at thepupil of the illuminator; and d. a uniformizer arranged between saidelements and the pupil of the illuminator.
 2. The profiler of claim 1wherein the distance between said elements is variable, to vary theradius of said annular intensity profile.
 3. The combination of claim 2wherein said distance between said elements can be reduced enough tocounter said divergence approximately at its source to keep saidintensity configuration from becoming annular.
 4. The combination ofclaim 1 wherein said upstream element has said concave conical surfaceand said downstream element has said convex conical surface.
 5. Thecombination of claim 1 wherein a mask is positionable at said pupilwithin said annular intensity profile.
 6. The combination of claim 1wherein said concave and convex conical surfaces have the same conicangle.
 7. The profiler of claim 1 wherein said elements are separated byan air gap.
 8. The profiler of claim 1 wherein said conical surfaces arearranged to confront each other.
 9. The combination of claim 8 whereinthe distance between said elements is variable, to vary the radius ofsaid annular intensity profile to accommodate characteristics of thereticle.
 10. The combination of claim 10 wherein said conical surfacescan be moved into proximity for countering said divergence to keep saidintensity configuration from becoming annular.
 11. The combination ofclaim 10 wherein a mask of variable size is positionable downstream ofsaid conical surfaces within said annular intensity profile.
 12. Thecombination of claim 1 wherein said refractive elements are faceted. 13.In an illuminator for a photolithographic projection imager, theimprovement comprising:a. a first refractive element arranged in acollimated region of an illumination path of said illuminator upstreamof a pupil of said illuminator so that a conical surface of said firstrefractive element diverges the illumination into a single beam havingan annular configuration of intensity; b. a second refractive elementarranged to receive diverged illumination from said first refractiveelement, and said second refractive element having a conical surfacearranged for countering the illumination divergence caused by said firstrefractive element, to fix the radius of the divergence of the singlebeam of said illumination; c. the radius of divergence of theillumination output from the second refractive element appearing as anannular intensity profile of illumination at the pupil region of theilluminator causing illumination with an annular intensity profile toilluminate a reticle that is imaged onto a wafer by an objective imagingsystem of the photolithographic projection imager; and d. a uniformizerarranged between said first and second refractive elements and the pupilof the illuminator.
 14. The improvement of claim 13 wherein said conicalsurface of said first refractive element is concave, and said conicalsurface of said second refractive element is convex.
 15. The improvementof claim 13 wherein said first and second refractive elements areseparated by an air gap.
 16. The improvement of claim 13 wherein adistance between said refractive elements is variable for varying saidradius of illumination divergence to accommodate characteristics of thereticle.
 17. The improvement of claim 16 wherein a minimum of saidvariable distance between said refractive elements results in saidsecond element countering the illumination divergence so that theconfiguration of illumination intensity does not become annular.
 18. Theimprovement of claim 16 including a variable size mask arranged forblocking illumination within said annular configuration of intensity.19. The improvement of claim 13 wherein said conic surfaces of saidfirst and second refractive elements have the same conic angle.
 20. Theimprovement of claim 13 wherein said conic surfaces of said first andsecond refractive elements confront each other.
 21. The improvement ofclaim 20 wherein a distance between said refractive element is variablefor varying said radius of illumination divergence to accommodatecharacteristics of the reticle.
 22. The improvement of claim 21 whereinsaid illumination divergence is substantially eliminated by moving saidconic surfaces into proximity.
 23. The improvement of claim 21 whereinillumination within said annular configuration of intensity is blockedby a mask.
 24. The improvement of claim 13 wherein said conic surfacesof said first and second refractive elements face away from each other.25. The improvement of claim 13 wherein said refractive elements arefaceted.
 26. An illuminator combined with a photolithographic projectionimager, the combination comprising:a. the illuminator having an opticalsystem for directing illumination along an optical axis of theilluminator upstream of a pupil of the illuminator so that an intensityprofile of the illumination at the illuminator pupil is directed to areticle that is imaged on a wafer by an objective imaging system of thephotolithographic projection imager; b. a diverging element arranged ina collimated region of the illumination path of said illuminatorupstream of the illuminator pupil for diverging said illumination into asingle beam having an annular configuration of intensity; c. a counterdiverging element arranged in said illumination path at a variabledistance from said diverging element for receiving said divergingillumination; d. said counter diverging element being arranged forcountering the divergence of said illumination and fixing the radius ofsaid annular configuration of intensity of the single beam as a functionof the distance between said elements; e. the annular configuration ofillumination intensity output from the counter diverging elementappearing as an annular intensity profile of the single beam of theillumination at the illuminator pupil and at the reticle so that theradius of the annular intensity profile accommodates characteristics ofthe reticle; and f. a uniformizer arranged between said elements and thepupil of the illuminator.
 27. The combination of claim 26 wherein saidelements are refractive and have faceted surfaces.
 28. The combinationof claim 26 wherein said elements are concentrically diffractive. 29.The combination of claim 26 wherein said elements are reflective andhave conical surfaces.
 30. The combination of claim 26 wherein saidelements are refractive and have conical surfaces.
 31. The combinationof claim 30 wherein said conical surfaces are concave on one of saidelements and convex on another of said elements.
 32. The combination ofclaim 31 wherein said concave and convex conical surfaces confront eachother.
 33. The combination of claim 32 wherein said counter divergingelement can be positioned for countering said diverging illumination sothat the illumination intensity profile does not become annular.
 34. Thecombination of claim 31 wherein said concave and convex conical surfacesface away from each other.
 35. The combination of claim 26 wherein saiddiverging element is refractive and has a concave conical surface. 36.The combination of claim 26 wherein said counter diverging element isrefractive and has a convex conical surface.
 37. The combination ofclaim 26 wherein said counter diverging element can be positioned forcountering said diverging illumination so that said intensityconfiguration does not become annular.
 38. The combination of claim 26including a variable size mask positioned to block illumination withinsaid annular configuration.